Vacuum tubes, the predecessors of transistors and diodes, are air tight chambers with cathodes and anodes. The air is largely evacuated from the tube, hence the name vacuum tube. The tube's cathode is held at a lower voltage than the tube's anode so that electrons are accelerated from the cathode to the anode. As electrons move to the anode, they collide with air molecules knocking even more electrons loose and thereby amplifying the number of electrons. In many tubes, the cathode is heated to produce thermionic electrons. In other tubes, photons are allowed to impact the cathode to cause the release of photoelectrons.
Vacuum tubes are rarely used in circuitry any more. They are, however, often used in light detection. Some tubes are so sensitive that a single photon can cause an electron to leave the cathode and induce a large avalanche of secondary and tertiary electrons that reach the anode. One type of photon sensitive tube is a flame detector tube. A flame detector tube is sensitive to the photons produced by flames.
In operation, a tube's anode and cathode are subjected to a constant and necessary bombardment of electrons and ions. The result is the etching and sputtering of the cathode and anode. To provide long tube life, the anode and cathode are often made from or coated with resistant materials such as tungsten and molybdenum while still being consistent with the demands for the proper work function. Similarly, the gas in the tube is chosen to be one that will not damage the anodes and cathodes too much nor react with other tube materials consistent with proper breakdown characteristics. Neon and a neon/hydrogen mix are often used as tube gasses because they are fairly light and nonreactive.
In the manufacture of vacuum tubes, a burn-in period is often required. When first produced, anodes and cathodes are rough. The rough surfaces affect the electric fields and result in inconsistent and occasionally even damaging electron flows and sputtering effects. Burn-in is a process in which the tube is run at an elevated voltage to sputter the surfaces smooth. The materials and gases used in vacuum tubes, however, are specifically chosen to minimize sputtering. Engineering decisions for extended tube life also cause long burn-in times. Some burn-in procedures must be performed before the vacuum tube is sealed. As such, there is ample opportunity for environmental gases to contaminate the inside of the vacuum tube. Systems and methods for contamination free burn-in of non-sealed vacuum tubes are needed.